Automated patient sample analysis devices have been developed to run various tests or "assays" for the detection of various biological substances and determination of various biological quantities. Fully-automated apparatus of this type typically employ a rotor or platter for receiving one or more cassettes or cartridges containing the necessary chemical reagents for analyzing a patient's sample, typically human blood, blood plasma, or blood serum. It is often necessary to separate whole blood cells from their blood plasma or serum medium so that subsequent reaction of the plasma with various reagents can proceed. Such a separation step often involves spinning a platter or rotor at a high speed, up to 10,000 RPM, to achieve the desired centrifugal force which separates the whole blood cells from the blood plasma. After such separation has been achieved, the plasma may then react with various reagents to produce, for example, conjugates having optically detectable labels or labels detectable by other means. Detection and quantification of the labels are thus indicative of a biological quantity to be recorded.
Often, incubation and agitation of the separated plasma with the suitable reagents are necessary steps in performing the assay. Precise control of assay cartridge temperature, agitation magnitude, and agitation time may be necessary to achieve repeatable assay results. It is therefore highly desirable to control the degree of agitation to which such cartridges or cassettes are subject to provide consistent test results. It is also desirable for purposes of efficiency to process one or more cartridges, each containing samples from different patients, simultaneously. Assuming that the rotor itself is balanced about its rotation axis, and further assuming that receptacles for the cartridges are positioned at regular angular intervals about the rotation axis, the rotor will remain dynamically balanced as long as a cartridge is received in each cartridge receptacle on the rotor, or as long as multiple cartridges are distributed symmetrically around the rotor. However, in a clinical setting it may be desirable to operate the analysis instruments with less than a full load of cartridges for the rotor. In this case, the rotor will not remain balanced unless "dummy" cartridges are inserted into the empty receptacles of the rotor, or when the cartridges are symmetrically distributed by the instrument operator, which may be impossible due to the fixed spatial relationship of the cartridge receptacles. In the absence of providing " dummy" cartridges or some other means for balancing the rotor, undesirable vibrations can develop which may interfere with the performance of the assays. For example, consider a rotor having a plurality of receptacles for assay cassettes, and further assume that each cassette weights approximately 10 g when loaded with the appropriate reagents and patient sample. Assume further that the center of mass of the cassette is positioned 9 cm from the rotation axis. At 5,000 RPM, the radial force exerted by the cassette on the rotor is approximately 55 lbs. If this force is not balanced by a counterforce, vibrations may develop which will undesirably agitate the received cassettes in an uncontrolled and unanticipated manner. In addition, the vibrations may detrimentally effect the structural integrity of the analysis device.
To overcome the above-described difficulties, at least one automated patient sample instrument manufacturer has introduced a passive system for counterbalancing a rotor having a plurality of cassette receptacles. As described in Clinical Chemistry31(9), 1985, a two-dimensional centrifugation system for desktop clinical chemistry is described which employs a rotor having a plurality of receptacles for assay cassettes. The receptacles are positioned at the periphery of a rotor at regularly spaced angular intervals. Associated with each receptacle is a weight which slides on a radially-directed track. The weight is biased to move inwardly towards the center of rotation when the rotor is not rotated. At high rotational speeds, the weights move radially outward under centrifugal force to provide a larger centrifugal force on the rotor than at times when the weight is positioned radially inward. When a cassette is received in a cassette receptacle, a mechanism prevents the weight from sliding outwardly. Although this device suitably suppresses undesirable vibrations in the apparatus by counterbalancing the rotor, this device requires that a sliding weight, spring bias mechanism, and associated locking device be provided for each receptacle of the rotor. Such a system is expensive to manufacture and undesirably reduces the reliability of the counterbalancing technique because each of the counterbalancing devices for each cassette receptacle must operate properly for the rotor to be counterbalanced.
Various techniques are known for balancing shafts on high speed rotating equipment. These techniques often involve the rotational movement of two or more lopsided or elliptical cams with respect to the shaft rotation axis in response to vibrations developed in the shaft. Such devices typically employ a vibration sensor which detects the magnitude of shaft vibrations. The cams are then rotated until the vibrations subside. Such a system is inapplicable to a patient sample testing instrument described above because it is necessary to prevent undesirable vibrations before they occur to ensure that the assay cassettes do not receive any agitation in addition to the programmed agitation which may be provided by the test instrument.
Therefore, a need exists for a self-balancing apparatus for a rotating centrifuge which utilizes a minimum number of moving parts, which is relatively inexpensive to manufacture, and which balances the rotor prior to high speed centrifugation of the cassettes.